The demand for wireless communications continues to rise. Broadband services are stretching the limits of spectrum. The Federal Communications Commission (FCC) has issued a notice of proposed rulemaking (NPRM) encouraging cognitive radio technology as a candidate to implement negotiated or opportunistic spectrum sharing.
Cognitive radio technology is an enhancement on traditional software radio design in which observations of the operating environment are combined with knowledge of the radio's available hardware and software capabilities to form decisions as to how to modify the radio's behavior to produce a desired level of performance.
Cognitive radio technology includes the capability of mobile nodes to alter transmission parameters based on knowledge of its environment. For example, a cognitive radio network can be a network of mobile communication nodes (hereinafter referred as nodes) in which a mobile communication node changes its transmission or reception parameters to communicate efficiently in order to avoid interference with incumbent users. This alteration of parameters is based on active monitoring of several factors in the external and internal radio environment, such as radio frequency spectrum, user behavior and network state.
Since current wide area networks such as cellular networks cannot meet projected demands, there is a need to create alternative wide area networks. Ad hoc is a promising networking technology that is targeted toward unlicensed spectrum.
Ad hoc networks are self-forming networks which can operate in the absence of any fixed infrastructure, and in some cases the ad hoc network is formed entirely of mobile nodes. An ad hoc network typically includes a number of geographically-distributed, potentially mobile units, sometimes referred to as “nodes,” which are wirelessly connected to each other by one or more links (e.g., radio frequency communication channels). The nodes can communicate with each other over a wireless media without the support of an infrastructure-based or wired network. Links or connections between these nodes can change dynamically in an arbitrary manner as existing nodes move within the ad hoc network, as new nodes join or enter the ad hoc network, or as existing nodes leave or exit the ad hoc network. Because the topology of an ad hoc network can change significantly, techniques are needed which can allow the ad hoc network to dynamically adjust to these changes. In ad hoc networks which lack a central controller, many network-controlling functions can be distributed among the nodes such that the nodes can self-organize and reconfigure in response to topology changes.
One characteristic of ad hoc network nodes is that each node can directly communicate over a short range with nodes which are a single “hop” away. Such nodes are sometimes referred to as “neighbor nodes.” When a node transmits packets to a destination node and the nodes are separated by more than one hop (e.g., the distance between two nodes exceeds the radio transmission range of the nodes, or a physical barrier is present between the nodes), the packets can be relayed via intermediate nodes (“multi-hopping”) until the packets reach the destination node. In such situations, each intermediate node routes the packets (e.g., data and control information) to the next node along the route, until the packets reach their final destination. For relaying packets to the next node, each node maintains routing information collected through conversation with its neighboring nodes. The routing information can also be periodically broadcast in the network to reflect the current network topology. Alternatively, to reduce the amount of information transmitted for maintaining accurate routing information, the network nodes may exchange routing information only when it is needed.
With this FCC ruling, cognitive technology coupled with ad hoc technology provides almost unlimited spectrum availability. With this coupling of technologies applied toward the creation of a wide area network solution, a significant problem is determining how to make this new wide area network solution as reliable as incumbent systems operating strictly within licensed spectrum. The first step toward solving this problem is overcoming propagation distances at difference frequencies. In a system with mobility, ad hoc networks must provide mechanisms to maintain alternate paths or routes between nodes of the network in order to maximize the reliability of the network.
Accordingly, there is a need for a method to enable traffic to be maintained over multiple hops of an ad hoc network when the alternate paths may reside on different frequency bands with different propagation characteristics.
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